Capacitors are important elements in many electronic devices, including integrated circuit devices. Many applications of the capacitors require that a capacitor is charged and discharged. However, the charging and discharging can lead to problems in the circuit. Conventional circuits for enabling the charging and discharging of a capacitor rely on sink/source devices, such as bias transistors, to control the amount of current flowing to or from the capacitor. The switch is controlled digitally by charge and discharge signals that dictate the flow of the current.
Conventional circuits have used charge and discharge paths to charge and discharge a capacitor including a bias device to dictate the current magnitude, a cascode device to give good power supply rejection and gain, and a switch to perform the switching. Because the bias device is turned off and on by the switching of this scheme, a large glitch occurs on the transition. Some conventional circuits have added matching dummy paths in parallel to the charge and discharge paths to shunt the current during the “off” phase without changing the state of the bias device, thereby reducing the glitch. However, there may still be charge injection associated with the parasitic capacitance of the switches into a node at a terminal of the capacitor when switching from a charge mode to a discharge mode or vice-versa. The charge injection depends on the size of the switch, which in turn dictates the amount of glitch induced in the node at the terminal of the capacitor. While making these switch devices small will reduce the size of the glitch, such a reduction is made at a cost of headroom for the bias and cascode devices because the switch resistance increases and thus they require more IR drop in the system. As transistor sizes continue to be reduced and voltage supply levels continue to drop, cascode devices are becoming more difficult to implement because of the reduced headroom.
According, improved circuits for and methods of implementing a charge/discharge switch in an integrated circuit is beneficial.